Electrolytic Capacitors: Borate Paper Wound with Formed Aluminium Foil and Vacuum-Impregnated (Danziger, US 2,107,780)

A dry electrolytic capacitor wound from aluminium foil and paper. The paper is first soaked in ammonium borate and dried, so the salt is already in its fibres. After winding, the bodies are soaked under heat and vacuum in borate dissolved in glycol, boxed and aged at 500 volts. Result: electrolytic capacitors. In the patent's tests these lost only 4.8 % of their capacity, against about 33 % for bodies made the ordinary way, and their power-factor loss rose 87 % on life test against 250 %. Source: H. I. Danziger and A. Di Giacomo, US 2,107,780 (1938). Unknown: how the anode foil is formed, which paper, the tab metal, and how the ammonium borate is made; the patent does not give them. Picture: Wikimedia Commons.